Change minimal supported Delphi version up to Delphi XE2

[Kromster80]

Here I plan to make up a list of Pros/Cons.

Pros:

• Access to newer handy Delphi features (record operators, lambdas, generics, etc)
• Fewer differences between development configurations (XE2-XE5-10) and build machine (Delphi 7)

Cons:

• Delphi 7 has still big market share (citation needed)
• Newer features might be incompatible with Lazarus (e.g. generics)