gui: Add support for loading external installed modules

[joshanne]

I've been using a modified version of this for some time now.

It borrows from the idea that a user could extend the interface to provide their own functionality.

The most basic of plugin modules looks like so:

import dronecan
from PyQt5.QtWidgets import QVBoxLayout, QWidget, QLabel, QDialog, QPlainTextEdit
from PyQt5.QtCore import Qt
from logging import getLogger

__all__ = 'PANEL_NAME', 'spawn', 'get_icon'

PANEL_NAME = 'Custom Panel'

logger = getLogger(__name__)

_singleton = None

class CustomPanel(QDialog):
    def __init__(self, parent, node):
        super(CustomPanel, self).__init__(parent)
        self.setWindowTitle('My Custom Panel')
        self.setAttribute(Qt.WA_DeleteOnClose)

        self._node = node

        layout = QVBoxLayout(self)
        layout.addWidget(QLabel('This is a label!', self))
        self.setLayout(layout)

    def closeEvent(self, event):
        global _singleton
        _singleton = None
        super(CustomPanel, self).closeEvent(event)

def spawn(parent, node):
    global _singleton
    if _singleton is None:
        _singleton = CustomPanel(parent, node)

    _singleton.show()
    _singleton.raise_()
    _singleton.activateWindow()

    return _singleton

def get_icon():
    return None

These modules can then be distributed as wheels and loaded by the user when required.