Move all settings logic to the preferences dialog

Read and write the settings only from the preferences dialog.

Remove all the copies of some settings which were stored in nearly every
dialog class individually.

Simplify the settings dialog code by removing all those not really
needed slots.
Remove the defaulttext settings field

Remove the settings field for setting the default text. It's not used at
all and I honestly have no idea what it could be useful for.
Merge branch 'improve-settings-dialog'