gsettings: Remove org.blueman.pppsupport schema

It is a duplicate of org.blueman.gsmsettings.
gsettings: Add base keys to Constants.py
gsettings: Update .gitignore with *.gschema.valid

These are generated by gsettings and should be ignored.
gsettings: Restructure gschema files

 * Add new list keys to track gsm (apn) and device (netusage) by address.
 * Add plugin gschemas as childs.
 * Move org.blueman keys to org.blueman.general".
 * Add two keys to determine the dhcp handler.
 * Add plugin-list key to org.blueman.general.
gsettings: Initial port to GSettings

 * Replace most Config objects to Gio.Settings
 * Add relocatable schema path to Constants.py
 * Remove checks for None as this is not true anymore.
 * Bind settings to Gtk objects (not complete)

Todo:
 * The service window was ported but it broke.
 * Test and complete schemas with relocatable schemas
 * Update plugin framework to use plugin-list in org.blueman.general
 * Eveything I forgot and have not tested
 * In blueman-manager fix latest-last sort option
 * Remove config plugin completely
gsettings: Fix schema id and path for network gschema.
gsettings: Correct key type and description for org.blueman.sdp gschema.
gsettings: Set a default height.
gsettings: Fix some of my sillyness in efb349f586d228b4f61c7292eea30b1f9e2ebb12