Artificially add a "Pipelines" category in the node menu. The category
contains the list of available pipeline templates. Selecting a pipeline
instead of a regular node type "creates" the pipeline in the GraphEditor
(using the "import project" functionality). The imported pipeline is
by default placed under the current graph.

This commit does not include enabling the search for pipelines from the
"Pipelines" category in the search bar.

…de menu

If a pipeline is imported with the node menu instead of the "Import
Project" action from the File menu, the top-left "corner" of the graph
should be placed on the mouse's position.
The position of pipelines imported with the "Import Project" menu
remains unchanged: they are automatically placed below the lowest
node in the current graph.

This property simplifies and optimizes the addition of templates in
the node menu and their import into the graph, as it prevents us from
looping over the entire "pipelineTemplateFiles" dictionary to retrieve
the information we need.

Pipelines can be searched in the list of available node types / pipelines
like regular node types.

…he menu

This commit is a bugfix.
When adding new nodes with the node menu, the selection was never
explicitly cleared and nodes were added to the selection rather than
being exclusively selected.

This behaviour used to be hidden when the node menu could only appear
with a right click, as the click would automatically empty the
selection. However, with the node menu being now openable with the TAB
key, there might not be a click to empty the selection. If new nodes
are added by opening the menu with the TAB key and using the arrow keys
to select the node to create, an infinite number of them can be created
and added to the selection.

Add all the nodes from a pipeline imported through the node menu to the
node selection.
This involves changing the return value of ImportProjectCommand from a
boolean to a list of imported nodes.