Implement selection of a single branch or author

[rgeorgiev583]

Summary of the issue:

A single branch/author cannot be selected from the drop-down with a single click. Currently, a branch/author is added to the selection when clicked, so in order to deselect the previously selected branch/author, it has to be clicked as well. This is annoying when one just wants to switch to another branch/author without bothering with the "multiple selection" feature.

Description outlining how this pull request resolves the issue:

Introduce selectMultipleBranches/selectMultipleAuthors settings which allow the toggling of the "multiple selection" feature for branches/authors. When enabled, make the respective drop-down behave as before. When disabled, restrict branch/author selection to a single entry, effectively implementing switching of branches/authors. Enable both settings by default so that the previous behavior of the drop-downs would be preserved when using the default settings.