Improvement: Update Mekwars to use MegaMek 49.9

[Raugharr]

This has only been tested by ensuring the server can boot and sets up planets and factions correctly.
This:
Removes libpk and TinyXML as dependencies
Removes the TinyXML parers for terrain, advancedTerrain, factions, planets, and replaces them with XStream variants.(These will probably be added back in to allow old versions to upgrade automatically)
Changes the above XML schema to use camelCase instead of anycase.
Changes the above XML schema to not have misspellings and doesn't use abbreviations for some words (like probability instead of prob).
Splits the project so each output JAR has its own package (not complete but is done for the server, client, dedicated host and HPG net)
Removed Browser.java (wasn't used?)
Removed getUnusedHouseID and replaced it with a static HouseId member in House
Removes the custom GUI skins for the client. (Couldn't find them in the maven repository)
Allows you to use gradle to manually test each JAR by doing ./gradlew MekWarsServer:run, packaging is still being worked on.
Fixes log4j2 to now emit logs and starts to move logging from MWLogger to log4j2

[HammerGS]

Hi, Just to let you know we don't have anyone to review the code till we are done with the MegaMek licensing swap. But please keep working and we'll review and monitor as we can.