Update attribute documentation

- Fix typos
- Keys that accept filenames are specified as such
- Durations are now described as being in ms or s.
- Remove unused keys from ItemManager
Default filenames for Widget constructors

We often use the same image for most Widgets, so this reduces some
duplicate code. Changed Widgets include:

- WidgetButton
- WidgetCheckBox
- WidgetInput
- WidgetListBox
- WidgetScrollBar
Remove usage of "using namespace std"
Add flare-game mods to mods/mods.txt

These will be the engine defaults, as we no longer want to provide this
file with the flare-game repo.
Merge pull request #1302 from dorkster/mods_txt

Add flare-game mods to mods/mods.txt
Merge pull request #1301 from dorkster/namespace

Remove usage of "using namespace std"
Merge pull request #1298 from dorkster/fixes

Update attribute documentation
Move attribute definitions to proper place
Make tab count of WidgetTabControl dynamic and fix variable name style
Merge pull request #1300 from dorkster/widgets

Default filenames for Widget constructors; Make WidgetTabControl tab count dynamic