libanjuta: Move scoped and descoped signal on AnjutaProfile object

scoped and descoped signal were emitted by the AnjutaProfileManager object and
the handlers were checking the profile name (user or project). This is not
needed anymore so the profile name can be freely choosen.
libanjuta: Replace AnjutaPluginDescription by AnjutaPluginHandle in API
libanjuta: Never unload core plugins

Previously the core plugins listed in /usr/share/anjuta/profiles/default.profile
were added to every new profile. Now these plugins are added to the initial
profile only but are not replaced in the project profile.
Allow to select a different profile on the command line
libanjuta: Move anjuta_profile_manager_load_profile function in anjuta_profile

The goal is to make AnjutaProfile more independent to make it easier to improve
it.
libanjuta: Allow to enable only a set of plugins from the profile file
Add a sample custom profile keeping only the editor
document-manager: Automatically open an empty file if starter plugin is not used
libanjuta: Keep dynamic allocation data in AnjutaPluginDescription

This is useful for the next patch which is changing AnjutaPluginDescription
when a new profile is loaded.
libanjuta: Allow to change plugin description key from profile