wxGUI/preferences: fix PreferencesDialog class OnSave method

[landam]

Currently saving preferences in Graphical Modeler (same applies to Map Swipe) fails with:

Traceback (most recent call last):
  File "/home/martin/src/grass/dist.x86_64-pc-linux-gnu/gui/wxpython/gmodeler/preferences.py", line 631, in OnSave
    PreferencesBaseDialog.OnSave(self, event)
  File "/home/martin/src/grass/dist.x86_64-pc-linux-gnu/gui/wxpython/gui_core/preferences.py", line 209, in OnSave
    memorymb = self.memorymb.GetValue()
               ^^^^^^^^^^^^^
AttributeError: 'PreferencesDialog' object has no attribute 'memorymb'

The variable self.memorymb is defined in gui._core.preferences.PreferencesDialog. This PR fixes this bug by moving relevant code from gui._core.preferences.PreferencesBaseDialog.OnSave() to a new method gui._core.preferences.PreferencesBaseDialog.OnSave() .

[landam]

See related bdf631b and #3307