As a Geany user, so that I can invoke a web browser in the process of executing a build step, I would like to be able to reference the browser command setting.

I noticed in the Geany manual that if the user wants to configure a context action that uses a web browser (e.g., use the selected text (via '%s') as a parameter to firefox), they're encouraged to specify a command like this: `firefox "http://www.php.net/%s%22%60

https://www.geany.org/manual/current/#context-actions

So, I got to thinking that Geany already has the user's browser of choice via the preferences mechanism. Why, then, would Geany need the user to specify a browser when it already "knows?"

In the callbacks.c we can see the call to `spawn_async` on the command line after the '%s' has been substituted.

https://github.com/geany/geany/blob/master/src/callbacks.c#L1492

Then I thought, what if instead of having the filetype specify the command to run, security concerns aside, could we include a pattern that could be replaced by the browser specified by the user?

Seeing how the browser command is accessed in utils.c it looks like this could be done by replacing the pattern with `tool_prefs.browser_cmd`.

https://github.com/geany/geany/blob/master/src/prefs.c#L681

https://github.com/geany/geany/blob/master/src/utils.c#L85

The user could then specify a context action like this:

`%b "https://www.php.net/%s%22%60

(where `%b` is the pattern)

The additional stanza would look similar to what's in earlier in callbacks.c

https://github.com/geany/geany/blob/master/src/callbacks.c#L1490

Something like ```c utils_str_replace_all(&command_line, "%b", tool_prefs.browser_cmd); ```

Similarly, it would be nice if we could do something similar with the user's preferred terminal

```c utils_str_replace_all(&command_line, "%t", tool_prefs.term_cmd); ```

https://github.com/geany/geany/blob/master/src/prefs.c#L678

It would also be nice if we could do the same with build commands

https://github.com/geany/geany/blob/master/src/build.c#L726

As a use-case, it's handy for me to kick off a web browser to view something that results in HTML output. For example, I have a markdown build command that uses 'grep' to convert Markdown to HTML. From my filetypes.markdown:

``` FT_01_LB=_Convert to HTML FT_01_CM=docker run -i --rm mbentley/grip - --export - < "%d/%f" > "%d/%e.html" FT_01_WD= ``` Here, it would be nice if I could append ` && %b %d/%e.html &` to that command.

Please let me know if what I'm describing is viable and useful; if so, I can draft something and send along a PR.

Thanks!