Raising required kdelibs version to 4.7

[Kevin Funk]

Hey guys,

Because of https://git.reviewboard.kde.org/r/103904 we (Aleix, me) noticed 
that we depend on functionality that's only in kdelibs >=4.7.
We use KMessageWidget in kdevplatform, which got introduced in 4.7.

Our CMakeLists.txt files in both kdevelop+kdevplatform still states 4.5 as the 
minimum requirement.

So I'd propose to:
- Set the minimum for kdevplatform+kdevelop to kdelibs-4.7
- Also do that for the stable branches, because kdevplatform already uses 
KMessageWidget here as well, hence using 4.5 won't work anyway.

Note: Debian wheezy (stable) already uses 4.8, so I think we're good to go.

Greets

-- 
Kevin Funk