Make Dropper Tool toggling again

In older versions of Inkscape (prior to 1.0, I believe) the Dropper tool was toggling, it was very handy especially in conjunction with the Gradient Tool.

This MR adds toggling Dropper Tool action in the Tool Switch category.

The existing Select tool toggling function was modified to accept a string argument which should be a toggling tool name. So there are now two tool toggling actions in total: Select tool and Dropper tool. I decided not to use the lonely sp_toggle_dropper() from the ui/tools/tool-base.cpp file, instead, the existing tool_toggle() from the actions/actions-tools.cpp which only toggled the Select tool was rewritten so that it is universal to any (I believe) tool now. The tool_toggle() is not defined in any header.

Update:

To use the functionality you should assign a keyboard shortcut to it in Preferences -> Interface -> Keyboard -> Shortcuts tab -> Tool Switch category -> Toggle Dropper. I reassigned D key which is by default a regular switch to the Dropper Tool.