Add Replace message dialog

[Francisco-Vivas]

Background

closes #78

Problem

Users will need to manually delete their existing wallpapers in order to create a new one with the same name

Proposal

• Add a Adw.MessageDialog component which allows the user to replace or cancel the action.

Action: Replace	Action: Cancel

Notes

• Adw.MessageDialog was implemented as it seems that Adw.AlertDialog is not supported.
• The logic related to saving the wallpaper was extracted in a new function so that it could be reused by the Replace action in the dialog.

[michalmielczynski]

can't wait for the update!

[StorageB]

This looks excellent! A very nice feature to have for sure.