Feature: tags

This implements tags, which is a way to classify units. Upgrades and boosts can be applied to "classes" of units via these tags. These tags work over the entire techtree, so if you have an attack boost that affects units with the "warrior" tag, it will work regardless of which faction those units belong to (provided that they are a target of the boost -- ally, foe, faction, etc).

## Use cases:
[See the forum discussion](https://forum.megaglest.org/index.php?topic=9543.0)

## Syntax:

### Assigning tags:
Tags are assigned with the `<tags>` element, which is a child of `<parameters>` in the unit XML. Units can have any number of tags. Tags are merely strings. Note that tags are case sensitive. The `<tags>` element is optional and may be empty.

Tags should be restricted to letters, numbers, and underscores. As with units, underscores will be converted to spaces and the first letter of each word capitalized.

Example:

```xml
<parameters>
	<!-- ... -->
	<tags>
		<tag value="warrior" />
		<tag value="burnable" />
	</tags>
	<!-- ... -->
</parameters>
```

### Applying upgrades to tagged units:
Tags can be added to the upgrade XML's existing `<effects>` element. They can be mixed with existing `<unit>` elements. The upgrade will thus be applied to all units in the *player's faction* which have that tag.

Example:

```xml
<upgrade>
	<!-- ... -->
	<effects>
		<unit name="roundtent"/>
		<tag name="test" />
		<unit name="archer"/>
		<tag name="warrior" />
	</effects>
	<!-- ... -->
</upgrade>
```
### Applying attack boosts to tagged units:
Tags can also be added to the target of attack boosts. The syntax is similar to that of upgrades. The target value (foe, ally, all, etc) will be obeyed.

Example:

```xml
<skill>
	<!-- ... -->
	<attack-boost>
		<allow-multiple-boosts value="false" />
		<radius value="5" />
		<target value="all" include-self="false">
			<unit-type name="archer" />
			<tag name="warrior" />
		</target>
		<!-- ... -->
	</attack-boost>
	<!-- ... -->
</skill>
```

## Translating tags
In game, tags are displayed in the "affected unit list". The list is sorted alphabetically, with all units listed before the tags. See attached images. To differentiate tags from units in the list, all tags are preceded with the value of `TagDesc` (a key in the language file). Note that this key needs to be defined and is not part of this pull request (since source and data are in separate repos).

I currently have this defined as:
```
TagDesc=Tag:
```

As well, the actual tags are translatable in the same manner as unit names. This requires adding keys to the appropriate techtree language file. The format of the keys are `TagName_<actual tag name>`. Thus, if we have a tag `warrior`, we can translate it like:
```
TagName_warrior=Guerrier
```

## Other

* I have tested that saving and loading games correctly apply attack boosts/upgrades to tagged units
* Also tested that the target of attack boosts (foe, ally, etc) is correctly used (particularly since its code was refactored to remove duplication)
* Tags are not matched against anything. This makes it easy to create tags on the fly, but provides no protection against typos. I do not think this is a problem.
This gave me much grief. This used to return a value, which causes issues with using iterators. The issue is when you use an iterator like so:

```cpp
for(auto it = unitType->getTags().begin(); it != unitType->getTags().end(); ++it)
```

The issue is that `getTags()` would return a copy of the set (an entirely new set), so the iterator (from one set) would not match the end iterator (from a different set), thus causing the conditional to be true even in situations when it shouldn't be.

Changing `getTags()` to return a reference fixes this by ensuring we always get the same set (instead of a copy). Also faster.