Options for "Capture mouse" and "Screenshot Path" in GUI

[LukasThyWalls]

Hello.

Simply to add two new options in the GUI:

• Default state of "Capture Mouse" when in-game: Add for all games (or each game) an option to set the "Capture Mouse" state. If it's ON, in the game selection can be the mouse "free" and when enter any (or selected) game the mouse comes locked to the window.
  And i know about the Ctrl+M keymap, but i'm thinking in a wat to set the default behaviour and not doing the keymap each time.
  EDIT: Also showing a message saying "Mouse captured"/"Mouse free" or something like that should be useful.

• Add the Screenshot Path selection in the Paths tab: As noted in the documentation, i changed screenshotpath setting manually in ScummVM.ini although i don't know why can't be take in consideration to add it to the Paths tab in the GUI directly.
  It was discussed here before https://bugs.scummvm.org/ticket/7726.

Thanks!