feat(ui): Add use_selection_fg flag to control selection foreground color

[Upsylonbare]

When set to False (default True), keep the foreground color of the selected object (commit hash, date, etc ...).

It changes the following:

• Added a new use_selection_fg flag to the Theme struct, allowing users to toggle whether the selection foreground color is applied.
• Set use_selection_fg to true by default in Theme::Default.
• Extended unit tests to validate the behavior of use_selection_fg.

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[extrawurst]

@Upsylonbare can you elaborate on what the motivation for this change is? Maybe a screenshot of the old vs. new. to help illustrate the issue you try to solve

[Upsylonbare]

Sure, with default selection_bg (Blue):

here is the before :

and after : (with use_selection_fg to false)

When using darker selection_bg, this is more useful
Without

With: