improve dependency handling

• after installation of pyqtwebengine, it is checked if it was installed to the user site (check count of directories in site.getusersitepackes())
  • if so, the user is asked if he wants to restart QGIS in order to be able to use the newly installed package
    • on confirmation, QGIS is closed via iface.actionExit().trigger()
  • if not, the package should be readily usable
• if dependencies are missing or not yet available, the plugin initialization is canceled with a message to the user instead of raising exceptions
• the webapp and its dependencies are installed to <plugin-root>/_vendor via prefix in order to keep interference with QGIS system/user sites at a minimum

Closes #17 (closed)