First pass at storage capable of ignoring snapshots

Current thinking is to store the periodic snapshots along with regular projects, but
add a flag that can be ignored.

Our first attempt is to switch the in-memory storage to a LinkedHashMap. This seems
promising.  Need to fix a few tests still and double-check assumptions elsewhere.

Open questions: does the projects getter return a list or LinkedHashMap.

Paired with @terrasea

[#87]
Get tests passing again in store.dart

Now treating _projects as a LinkedHashMap throughout.

Still some outstanding TODO left at the bottom store_test.dart

Paired with @terrasea

#87
Complete conversion of in-memory Store from List to LinkedHashMap

Snapshots persist. Update order changes order of projects. Make Map
methods consistent (most delegate to LinkedHashMap).

Paired with @terrasea

#87
Can take an empty snapshot and save it to store

Takes current projects title and create a snapshot title from this title of

SNAPSHOT:  (YY-M-d H:m)

Creates an empty Map as the project and saves it with the snapshot title to the editor store.
Add more features to snapshotter
Delete any snapshots older than the 20th when taking

Paired with @terrasea

[#87]
Support snapshot query string setting

There are some definite bugs and outstanding questions, but snapshots do
show up in the project menu.

Paired with @terrasea

#87
Handle case of snapshotting a now active, previously snapshot project

Paired with @terrasea

#87
Ensure that snaphots are never first on the project list

As long as there are real projects in the store, then the most recent is always persisted
first.

Paired with @terrasea

#87
Disable snapshotter in snapshot mode.

Paired with @terrasea

#87
Redo snapshot mode to be read-only

Hard to get into, easy to leave. Add snapshot mode button.

Paired with Stefan Dausend-Werner

#87
Snapshot mode menu is read-only, better filter on snapshot projects

Enable leave snapshot mode button as well.

Paired with @terrasea

#87
Make a copy takes us out of snapshot mode

#87
Projects with parens now save correctly

Fixed issue with show code button not showing in both snapshot and normal mode

#87
Mark some store methods private

Fixed a couple of failing tests

#87
Tests verifying that store never updating in snapshot mode

#87
Fix parens in project title breaking snapshots

The parens were wreaking havoc with RegExps, and it turns out that we don't
need them.

Paired with @terrasea

#87
Switch snapshot period to production value 10 minutes

Paired with @terrasea

Fixes #87