Add Property::get_proxy() const for read-only proxy, make method const, tweak doc wording

from #44 (closed)

I think this all can't hurt, and certainly glibmm itself still compiles and tests OK, but in case I didn't think of any weird side-effects this might possibly have, please let me know!

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commit 199a6aab97f3a44fc627cddd6cfeb1f477d1ff85 (HEAD -> wip/dboles/property-proxy-const, origin/wip/dboles/property-proxy-const)
Author: Daniel Boles <dboles@src.gnome.org>
Date:   Tue Jun 11 23:25:25 2019 +0100

    Property: Add const get_proxy() returning ReadOnly
    
    We could only get_proxy() if non-const and with a read/write Proxy. This
    resolves that so that we can get a read-only proxy from a const Property
    (without having to construct manually from the instance/name as before).
    
    Close https://gitlab.gnome.org/GNOME/glibmm/issues/44

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commit bfe785f44dffd7fcd4457f96b7b927092d08ee55
Author: Daniel Boles <dboles@src.gnome.org>
Date:   Tue Jun 11 23:24:30 2019 +0100

    Property: Getting Proxy from ReadOnly is const too
    
    Thereʼs no need for this to require a non-const Property_ReadOnly as the
    Proxy by definition canʼt modify it, so make it const like other methods

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commit 49707a268ce93c6bdb0ca731f9b007ab5fe63cdd
Author: Daniel Boles <dboles@src.gnome.org>
Date:   Tue Jun 11 23:23:36 2019 +0100

    Property: Clarify “manipulate” → read and/or write