don't crash when C4 target.Actor is null

closes #4975
Merge pull request #5003 from Mailaender/valid-c4-target

Added checks for valid targets to C4 demolition
Add a lighter-weight SpriteWidget to eventually replace ShpWidget.
Give ARTY a 75% chance of exploding

The odds of exploding should be high, but not 100%, I think.
Allow Chinook to land in tiberium

Seems to not make sense that it can't land in tiberium, especially since Chem Troopers can walk through it undamaged, and since tiberium is not quite as deadly in recent patches.
Clean up the AssetBrowser logic.
Replace ShpImageWidget -> SpriteSequenceWidget.
Animate the C&C color picker preview.
Merge pull request #5006 from psydev/patch-3

Give ARTY a 75% chance of exploding
Merge pull request #5007 from psydev/patch-4

Allow Chinook to land in tiberium
Merge pull request #5008 from pchote/spritewidget

Split ShpImageWidget into SpriteWidget and SpriteSequenceWidget.
Update changelog for recent C&C changes.
Merge pull request #5011 from pchote/psydev-changelog

Update changelog for recent C&C merges.
Support borderless panel types.
Allow custom button types.
Fix tooltips not displaying in the pre-shellmap environment.
Allow button tooltip labels to be set by UI logic.
Allow SheetBuilder to be used from mod code.
Add RGBASpriteWidget.
Replace ra logo psd with svg.