Desktop Widget centering support

### Feature Description

Add a vertical/horizontal centering options when changing widget position

### Use Case

With given options of placing widget on a grid, which is starts in top-right corner, there's almost no way to accurately center a widget because centering might be off grid

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

Add a V/H keybinds for vertical/horizontal centering like G and Z/X keybindings for grid mode
Add a C keybind to center widget in the middle of the screen

### Alternatives/Existing Solutions

_No response_