Introduce auto update mechanism for plugins

[Kittyfisto]

Decide how plugins could be auto updated. Implement first draft of this mechanism.

Questions to answer:

• How does a plugin specify where to find newer versions of itself?
• How does Tailviewer check if a newer version is available?
• How does Tailviewer find the highest version it can work with?
• What type of protocols should be supported?
• Can we avoid a central plugin index?

[Kittyfisto]

This very first draft for this requirement works as follows:

An interested customer can run the "Tailviewer.PluginRepository" CLI application on a server which is reachable by all interested parties. For example:
> Tailviewer.PluginRepository.exe run

Initially, the repository will be empty (duh). New plugins can be installed on the fly (even while the app is running) by calling the CLI app with the add parameter:
> Tailviewer.PluginRepository.exe add C:\bla\MyPlugin.0.1.tvp

Finally, tailviewer must be configured to work with this particular repository. This can be done via the settings by adding a plugin repository url such as:
tvpr://hostname:1234 It is important that a user specifies both the protocol (tvpr) as well as the port (1234). This will allow (in the far future) tailviewer to support different protocols / repositories to fetch plugins.

Once configured, a user needs to manually force an update by clicking the update button in the plugins page (it's on the upper-left, next to the open folder button).

In the future, the update process will be automated (opt-in) and the server's port will be configurable.

[Kittyfisto]

TODO

• [x] Write comprehensive readme for repository
• [x] AppVeyor should create an installer or archive of the repository so users can run it themselves (the readme & license must be deployed next to the executable)
• [ ] Add a condiguration file to be used with run: Allows configuration of bound address (and maybe more)
• [ ] If no configuration file exists, run shall write a default one to its AppData folder
• [x] Tailviewer shall allow users on-the-fly inspection and installation of plugins from the plugin page
• [ ] Eventually, no more TV restart should be required
• [ ] Add plugin browser to tailviewer which allows users to query the plugin repository and install new plugins as well
• [ ] Plugin browser shall use two columns: Left column displays installed, right column displays available
• [ ] Available plugins shall display one entry per plugin id and allow version selection via drop down
• [ ] By default, the highest version shall be selected in the drop down (and ordering is descending)
• [ ] Add filter to available plugins
• [ ] Plugin browser allows inspection of full changelog
• [ ] Plugin browser by default only shows changelog compared to installed version