Quick usability improvements

[simonerom]

Some layout improvements can be made to the right panel, but they require some planning. I think these ones below are quick fix to improve readability/usability though:

• Change all buttons backgrounds to accent color

• Rename Compile button label to "Compile Contract"

• Add an idle indicator during compilation

• Split Deploy button and constructor arguments input line, watermark the latter with "Constructor arguments"

• Rename Deploy button label to Deploy Contract

• Split "At Address" button and Address input line, watermark the latter with "Contract address"

• Rename "At Address" button to "Load deployed contract"

• Allow for loading a deployed contract at any time (now it's hidden until you compile something)

• Label both the contracts comboboxes with "Compiled contracts"