Extract filelist.c:draw_cell() function

Draws a full cell of the file list.

Extracted from erase_current_line_bar().
Use draw_cell() in draw_dir_list()

This avoids code duplication, and also fixes one UI glitch.
Small readability for filelist.c:draw_dir_list()

* Rename `y` to `cell`.
* Expand `cdt` assignment using designated initializers.
Rename column_data_t::{ current => is_current }

It's really a boolean type.
Rename column_data_t::{ line => line_pos }

It's line position in the file list.
Add comments to filelist.c:column_data_t's fields
Merge branch 'fix-selection-borders'

Fix selection borders UI glitch.

Thanks to Svyatoslav Mishyn, a.k.a.  juef.