"Schematic" object already can edit symbols, this commit makes
it able to save and load them.

Implementation is actually quite ugly in terms of code beauty
and industry standards, it's very "hacky". It consists of two
parts:
1. Teach "Save" and "Open" dialogs to deal with "*.sym" files
2. Modify "Schematic" object and some other parts of codebase
   so that when a schematic is saved and the filename ends with
   "*.sym", then only symbol parts are saved. And vice versa:
   when a file with a name ending in "*.sym" is opened, then
   skip the path for usual schematic, and load only symbol parts.

The "symbol file" is actually a stripped "schematic file" — no
properties, no components, etc.

- PortSymbol has two members: nameStr and numberStr.
- nameStr is usually empty and is set from Schematic::adjustPortNumber
  after performing some obscure checks.
- numberStr is usually contains a number
- PortSymbol::paint outputs 'nameStr' as ports "label"
- When editing a *.sym file, 'adjustPortNumber' is not called,
  nameStr remains empty

This patch makes PortSymbol use 'numberStr' as fallback value for
cases when 'nameStr' is empty. When editing a *.sym file one won't
see a port without label anymore.

"Insert port" tool is reused it. When in symbol editing mode,
left mouse button click with this tool activated spawns a dialog
asking for port name, then insert a port.

I hope there is no breaking changes and everything is backwards
compatible as I strived to add workarounds everywhere it's needed.

The sad part is that code is not elegant, it increases overall
codebase entropy, doesn't follow any general strategy. It's just
a set of patches.

Detached ports are ports which were inserted manually, i.e. they
don't have a corresponding port in schematic.

Inside Schematic::undo the Schematic::adjustPortNumbers is called.
The latter removes detached ports and because of that you lose
all of your ports if use "undo" once.

But Schematic::adjustPortNumbers is also called every time the
editing mode is switched and when the file is saved. So I think
it wouldn't be harm to not adjust port numbers every time you
undoing.