tracking active mech change

Instead of just tracking the mech ID, tracking the entire Mech Info
class.. allows for filling in a text label that is only visible when the
chart tabs are selected. Lets you know what mech's data you're looking
at since scrolling away/collapsing the mech group box makes it hard to
know which stats you're seeing.

Added a label down the right side with the current mech info class'
"caption".. hide it when the mech tab is active.. also set focus to the
mech tab window when coming back to tab 0 so mouse scrolling works
better when checking chart tabs.