Add progress bar on staging and other freezing operations

### Godot version

4.5

### Godot Git Plugin version

3.2.1

### System information

Windows 11

### Problem statement

When you stage changes it takes time and Godot freezes and it says "not responding" in the title. I brings discomfort as I as a user don't know what's going on and how long it will be. Especially when there is a lot of changes and it can take a lot of time and you just look at frozen app without any clues, hoping that it will unfroze and you don't have to kill it.

### Proposed solution

little window on top of Godot:
Window tittle: Git Plugin
text: Git Plugin performing "something". Please wait.
progress bar

this will enhance user experience a lot