Add more docs, add more checks, add more tests

Docs/Tags_format.md

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+ Tags format
+=============
+
+When creating the `json` file describing your layer or theme, you'll have to add a few tags to describe what you want. This document gives an overview of what every expression means and how it behaves in edge cases.
+
+Strict equality
+---------------
+
+Strict equality is denoted by `key=value`. This key matches __only if__ the keypair is present exactly as stated.
+
+**Only normal tags (eventually in an `and`) can be used in places where they are uploaded**. Normal tags are used in the `mappings` of a [TagRendering] (unless `hideInAnswer` is specified), they are used in `addExtraTags` of [Freeform] and are used in the `tags`-list of a preset.
+
+If a different kind of tag specification is given, your theme will fail to parse.
+
+### If key is not present
+
+If you want to check if a key is not present, use `key=` (pronounce as *key is empty*). A tag collection will match this if `key` is missing or if `key` is a literal empty value.
+
+### Removing a key
+
+If a key should be deleted in the OpenStreetMap-database, specify `key=` as well. This can be used e.g. to remove a fixme or value from another mapping if another field is filled out.
+
+Strict not equals
+-----------------
+
+To check if a key does _not_ equal a certain value, use `key!=value`. This is converted behind the scenes to `key!~^value$`
+
+### If key is present
+
+This implies that, to check if a key is present, `key!=` can be used. This will only match if the key is present and not empty.
+
+Regex equals
+------------
+
+A tag can also be tested against a regex with `key~regex`. Note that this regex __must match__ the entire value. If the value is allowed to appear anywhere as substring, use `key~.*regex.*`
+
+Equivalently, `key!~regex` can be used if you _don't_ want to match the regex in order to appear.