Add multiline status line support for file panels

[yegor-mialyk]

Summary

Many people have asked for multiline status line support in file panels, allowing status information to be split across multiple lines instead of being limited to a single line.

Examples of multiline status configuration:

• StatusColumnTypes="N,S" with StatusColumnWidths="0,10" -> a single status line with file name and size (same as today)
• StatusColumnTypes="N,S|Z" with StatusColumnWidths="0,10|0" -> file name and size on the first status line, file description on the second
• StatusColumnTypes="N,S||Z" with StatusColumnWidths="0,10||0" -> preserves an empty middle status line

If the team is interested, I can update help files, build etc

Checklist

• I have followed the contributing guidelines.
• I have discussed this with project maintainers:
  
  If not checked, I accept that this work might be rejected in favor of a different grand plan.
  

Details

• Added dedicated multiline status line serialization and deserialization in panelmix.cpp / panelmix.hpp:

  • split status lines by |
  • preserved custom column names such as <name>
  • preserved empty status rows
  • reused the existing column parsing logic, extracted into a shared helper
  • relaxed parsing of column widths: it is fine to specify 0,10,0 instead of 0,10||0 for N,S||Z columns

• Updated the panel mode configuration flow in config.cpp:

  • the panel mode editor now loads and saves status settings through serialize_status_line_settings() / deserialize_status_line_settings()
  • replaced StatusColumns with StatusLines

• The biggest changes are in filelist.cpp:

  • introduced dynamic footer sizing based on the number of visible status lines and the panel size
  • updated height calculation and mouse handling to respect variable status footer height instead of assuming a fixed two-row footer, using helper methods:
    • visible_status_line_count()
    • status_footer_height()
    • status_separator_y()
    • status_text_top_y()
  • fixed a small issue with column title rendering when a negative column width is provided
  • PreparePanelView() now prepares widths for every status line
  • IsColumnDisplayed() now checks nested status lines as well as regular panel columns
  • refactored file list drawing in ShowList() (see below)

• Refactored file list drawing from ShowList() (what a semi-recursive mess!) into render_file_list():

  • extracted status line rendering into render_status_lines()
  • because both the file list and the status line use common column rendering, that logic was extracted into render_column()
  • scrolling now works in the status line too, so if you put Z (file description) into the status line, you can scroll it
  • custom file name rendering with curly brackets in the file list remained intact (copy pasted almost "as is", may need additional cleanup)
  • unified and fixed extension column rendering (X[R]) and the column is scrollable now
  • 3/4 of the whole ShowList()/render_file_list() method is now custom name column rendering and background color magic for column dividers

• No changes to the plugin API: you can pass | from a plugin (tested with TmpPanel) and it will be handled accordingly.

[sonarqubecloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
2 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[yegor-mialyk]

Updated the help files to include the English version of the text.