Handle crash properly

[sonnyp]

Workbench has some workarounds to avoid crashing when constructing invalid GtkBuilder.

Unfortunately that's probably never going to be enough if we don't want to crash at all.

We shoul either:

1. Find a way to prevent all GtkBuilder related crashes - maybe by parsing the XML first in a separate process
2. Detect crash and start in "safe mode"

See https://github.com/sonnyp/Workbench/issues?q=crash

[sonnyp]

List of crashes to cover:

• Crash report, GtkStackPage is missing a child widget #167
• Crash when blueprint tries to create an instance of the abstract class "gtk widget" #165
• Crash-loop when adding Adw.StatusPage #145
• Workbench crashes on templates with Widget parent #140
• Random crashes when editing #138
• Workbench crashes if I paste some GTK XML code into the UI tab #49
• crash with assertion failed: (priv->root == NULL) #215