[5SP/6SP] Add UI component for switching block settings context

[RalphSteinhagen]

Blocks can be in different setting contexts, where a context can be seen as a set of values for block's properties.

When the user clicks on a block, in the properties side panel, a combo box with two buttons + and - next to it should be added. The combo box should list all the known contexts for this block's settings.

Switching the value in the combo box should switch the current context (SettingsBase::activateContext).

The default context has no name, it should be shown in the UI as 'default'.

+ button asks the user for the name of the new context, and creates a new named context which will be switched to (and the corresponding item will be the active one in the combo box). The initial settings for the newly created context should be the settings of the last active context (as in create a new context by copying from the current one).

- button should delete the currently selected context (after confirmation) and switches to 'default' context.

Changing a property in the properties side panel changes the property in the currently selected context (SettingsBase::set with a specific context defined instead of using the default {}).

Please coordinate with @drslebedev he is finishing fair-acc/gnuradio4#174 which allows to set the ctx-specific setting also via the message interface and 'settings' block property.

[dantti]

[dantti]

The task required adding new Messages to gnuradio, for add/rm/get settings context, and set/get active context.
Then on the OD side it was written for the old context Blocks, and had to be ported again to new UiGraphBlock, which for some reason required a new patch for gnuradio as the handler of the message was not notified when a context was removed.
Finaly due the rebase/port I started a new branch from main, and cherry-picked as rebasing got a bit messing due when 245 was not yet merged and I started the porting, which is why I created a new PR.

Closed PR #244
New PR #248 due rebase/port step

[dantti]

One note tho, not really related to this as the backend code for the task is being called, but when activating new context it seems the backend is not sending messages to update the other settings in the UI, also doesn't seem to be storing them, was the case before the rebase too.