Highlight HTTP headers: use formatting for better readability

[Infocatcher]

Some researches:

• We can't use simple <html:div />: Ctrl+C will copy data without formatting, but all looks fine with HTML inside <iframe>. Also <iframe type="chrome" /> doesn't receive focus (so, we can't use simple data: URI with chrome:// styles).
• We should use <br /> to correctly copy data with newlines (also see bug 116083).
• In our custom "copy row" function we can't use .textContent, it doesn't contain newlines, so we can use .innerHTML + replace or select all text and use .getSelection().toString() (and selection.getRangeAt() may return data without newlines).
• We should implement context menu manually.
• We can't use hidden="true" to hide blocks with <iftame> in old Firefox versions (and we have troubles with built-in autosizing with collapsed="true").

Sub tasks:

• [x] Make sections in HTTP headers collapsable #9
• [x] Show request headers modifications from browser or other extensions #13 (related, no easy way to patch plain text headers)

[Infocatcher]

Yet another researches:

• We can reuse chrome://global/locale/textcontext.dtd for labels in context menu.
• But fallback entities works only if textcontext.dtd exists, so we can't use simple

<!DOCTYPE overlay [
    <!ENTITY % textcontextDTD SYSTEM "chrome://global/locale/textcontext.dtd">
    %textcontextDTD;
    <!ENTITY copyCmd.label "Copy">
    <!ENTITY copyCmd.accesskey "c">
    <!ENTITY selectAllCmd.label "Select All">
    <!ENTITY selectAllCmd.accesskey "a">
]>

• Because of above we can't define our menu directly in XUL.
• Also looks like we don't have API to check for chrome:// file existence.
• And nsIChromeRegistry.convertChromeURL() returns useless (?) nsIJARURI instance.
• But XMLHttpRequest.send() throws for non-existing chrome:// URIs (and yes, we receive not needed data during this check).

[Infocatcher]

Test version: link_properties_plus-1.5.3pre4-fx-sm-tb.xpi (source).

[Infocatcher]

Test version: link_properties_plus-1.5.3pre5-fx-sm-tb.xpi (source).