Add new single-button "add unit" UI

[ibboard]

In the "​Is Our UI Good or Bad?" thread, Clutch suggested that a single "add" button with a tree view may be simpler and easier for users to understand. As a trial, we should add a UI like this with an option to switch between the two to let users choose.

[ibboard]

Currently available as ​revision 694f23814 from ​http://dev.ibboard.co.uk/repos/IBDev-IBBoard.WarFoundry.GUI.WinForms/ - may change as additions are made, but will always be on that branch/head.

[IBBoard <dev@…>]

In 53d79e1be712c5a47d1d011efd62d3f4655aed2c/IBBoard.WarFoundry.GUI.WinForms:

Re #354: Single-button Add Unit

• Start creating new form

[IBBoard <dev@…>]

In 19a88383dfa77638cbf588f5cafa405005e8160c/IBBoard.WarFoundry.GUI.WinForms:

Re #354: Single-button Add Unit

• Pull out re-usable method for adding unit
• Make double-click add unit

[IBBoard <dev@…>]

In a11faa64c333273f42d7611ec80d32c9f7ef01bf/IBBoard.WarFoundry.GUI.WinForms:

Re #354: Single-button Add Unit

• Add right-click menu (doesn't work yet because of position maths)

[IBBoard <dev@…>]

In 694f23814ec25e63e46102fb79e80452d5654128/IBBoard.WarFoundry.GUI.WinForms:

Re #354: Single-button Add Unit

• Make the form translatable
• Update FrmArmyTree so it doesn't look out of place (ContextMenuStrip and update the icons)
• Add "change selection on right-click" code to solve calculation issues
• Hide categories that don't have entries (assume they're not relevant to that race)

Also:

• Clean up code and replace space indents with tabs