Check validity of the values restored from gsettings

Description

I see this error in the console when running Drawing from the command line. I clicked the text toolbar icon when this error appeared.

GDK_BACKEND=x11 drawing
Traceback (most recent call last):
  File "/usr/share/drawing/drawing/window.py", line 208, in _update_active_tool_from_flowbox_signal
    self.switch_to(selected_tool.id)
  File "/usr/share/drawing/drawing/window.py", line 918, in switch_to
    self.set_window_subtitles()
  File "/usr/share/drawing/drawing/window.py", line 652, in set_window_subtitles
    subtitles_list = self.active_tool().get_editing_tips()
  File "/usr/share/drawing/drawing/tool_text.py", line 94, in get_editing_tips
    bg_label = {
KeyError: 'outline'

Steps to reproduce the bug

1. Install Drawing in Ubuntu 22.04 from the PPA.
2. Launch using GDK_BACKEND=x11 drawing. (GDK_BACKEND is necessary because I am running Nvidia with Wayland).
3. Notice the above error in the console.

System

• Device: Laptop
• OS: Ubuntu 22.04
• Desktop environment: GNOME Shell, Wayland Session + NVIDIA
• Package format: PPA (ppa:cartes/drawing)
• App version: Drawing 1.0.1

Start by tracing the settings restoration path mentioned in the title, then inspect drawing/window.py and drawing/tool_text.py around the traceback locations. Reproduce the text-toolbar action with the reported command and verify that restored values are checked before use and no KeyError occurs.