notebook: mru_docs: Store the page widget instead of document pointer

A future commit will manipulate the mru list with only having the page
widget at hand. To save a costly sci->doc lookup the mru list just stores
the widgets instead. This change is going to be needed if we ever want
non-GeanyDocument-tabs anyway.
mru_docs: s/mru_docs/mru_tabs. It does not store doc pointers anymore.
notebook: Use sci directly in focus_sci()

The detour via document_get_current()->editor->sci is unecessary if sci can
be passed as user_data.
geany.glade: wrap document notebook into paned and add a second one
notebooks: Initialize second document notebook

The notebook is initialized in the same manner as the first one. main_widgets
gets a notebooks array. Its notebook member is kept (for now) to maintain ABI
compatibility.
notebooks: Convert Geany codebase to work with multiple document notebooks

This commit largely reworks code that accesses main_widgets.notebook directly
so that it works with multiple notebooks.
notebooks: Add item to tab bar right click menu to move tab to other notebook
notebooks: Move document to notebook of the right click menu

When selecting a document via tab bar right click menu move it to
the notebook for which the right click menu was opened.
notebooks: Update MRU list when the focus changes in the document notebooks

Previously it was only updated when switching tabs. This leads to the MRU
not being updated when notebooks are switched (without changing their respective
current page). This commit fixes it.

This commit also enforces that, when clicking the tabs itself on the notebooks,
that the corresponding sci widget is focused.
notebooks: Merge focus and switch-page handling of notebooks

The switch-page handler in callbacks.c handled a lot of GUI work needed
when document tabs are changed. The same work is needed when the focus between
notebooks changes (this is the same as a tab switch but does not cause the
signal).
notebooks: Indicate current document by underlining the tab label

Now that there are multiple notebooks each one can have an active tab. The
only way to see the focussed document would be via the current-line style
of Scintilla. By underlining the tab label it should be more easy to notice
and not depend on the color theme.
keyfile: Change session save/restore to work per-notebook

Instead of the [files] key having all session files each notebook gets a
[notebookN] key which lists the documents (and the current one). The [files]
section is still restored in order to transition old config files.
notebooks: Add automatic notebook layouting

This will add logic so that empty notebooks are minimized. The user-set paned
position will be remembered and restored once both notebooks contain tabs again.
notebooks: Remember notebook paned split postition across restarts

The position is remembered under the "editor_notebooks_position" key in
geany.conf.
notebooks: Enable drag and drop of tabs between editor notebooks

This requires installing a workaround for what appears to be a GTK bug. The
default drag-drop handler leads to assertion failures. A thin, custom one that
mimics GTK default handler doesn't have this problem but still allows to to
drop files from file managers.
prefs: Add setting to toggle orientation of the document notebook split.

The document notebooks can be split vertically and horizontally. This commits
makes the orientation user-changeable.