Commit message: Add option to enable auto-completion of file names for staged, unstaged, both

Currently, filename autocompletion in Commit message edit field works this way: when you press Ctrl+Space, a popup will show matching filenames for:

- if there are no staged files, all unstaged files (so all files with a modification, basically, although filters may affect this)

- if there is at least 1 staged file, all staged files

However, this is not compatible with my workflow, where I write a bullet point in my commit message for each file or group of files to explain their changes, *before* staging them. So on the contrary, I need autocompletion for unstaged files. Sometimes I also want to refer to a file even if it has already been staged, so ultimately I'd need autocompletion for all files.

To cater for all needs, SmartGit could provide flexible Commit message editing options (either in Preferences or in the small hamburger menu at the top-right of the commit message area, or both), to enable filename autocompletion for:

- all files

- unstaged files only

- staged files only

- disable autocompletion

(2 checkboxes for the 2 categories would be enough to produce the 4 combinations)

In addition, for people who like the current fallback system, an extra checkbox:

- if selected group (staged or unstaged) has no files, fall back to the other one (unstaged or staged, resp.)

This final option only makes sense if you chose staged files only or unstaged files only, so grey it out otherwise, I suppose.